Reporters, opinionators, and analysts are right to have limited hopes for Cancun but dangerously wrong if they think the meeting is unimportant. Last year's talks produced the "Copenhagen Accord," a political agreement that was roundly savaged. Yet the accord is more valuable and important than most assume—and its future is at risk in Cancun. If negotiators let it die, as many privately wish, they will not get something closer to their ideal; they will get nothing.

The Copenhagen Accord did three important things. It established a series of global benchmarks against which countries' efforts to confront climate change could be judged. These include the goal of preventing global temperatures from increasing to more than 2 degrees centigrade above pre-industrial levels, and a goal of raising "up to $100 billion" by 2020 to help developing countries address climate change. It required all but the poorest countries to present national plans for curbing emissions; more than 100, including China and India, have followed through. And it committed countries to transparency, so that other nations could assess whether they were making progress.

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Despite these accomplishments, the accord was viciously attacked for what it lacked. Some focused on the fact that it was not a legally binding treaty. The line between gentlemen's agreement and hard law, though, is fuzzy in the world of international affairs. The Kyoto Protocol was legally binding, yet Canada has grossly violated it without penalty; China, meanwhile, has technically adhered to the protocol, but only because it required nothing of Beijing. There is no reason to assume that a wise political deal cannot be more effective than an unambitious but legally binding one.

Others have emphasized that the emissions-cutting pledges that countries made as part of the Copenhagen Accord are not enough, collectively, to meet the 2-degree target. But the pledges improved upon the status quo, and there is nothing in the accord that would prohibit countries from strengthening their efforts in the future. The Copenhagen Accord is surely insufficient by itself, but that does not make it unworthwhile.

Indeed, most of the attacks on the accord rest on a ridiculous comparison to a perfect agreement that will never exist. The political deal brokered by national leaders at Copenhagen was about the art of the possible.

Now there is a real possibility that the accord will effectively die at Cancun. The failure of cap-and-trade in the United States has sapped the influence of the original agreement's biggest backer. Fiscal pressures have made it difficult for wealthy countries to move toward the $100 billion goal. China has had buyer's remorse from the moment that it agreed to the accord and has been playing hardball on efforts to flesh out its transparency requirements. Those who would like to see the accord become irrelevant are aided by the broad sense that it doesn't really matter.

U.S. negotiators need to gather their allies and make a strong push to reinforce the accord and each of its key components. They should reiterate the United States' intention to cut its emissions by 17 percent from 2005 levels by 2020, something that is still achievable (though very difficult) without passing cap-and-trade legislation in the next few years. They should press the conference to endorse the recent report of the U.N. secretary-general's High Level Panel on Climate Finance, which outlined several paths to the $100 billion goal, as a worthy guide to moving forward. To that end, U.S. negotiators should also pledge to protect climate-assistance funding from upcoming congressional fights over foreign aid. And they should insist that China make concrete concessions on international transparency, which are essential to building trust in the future.

Negotiators, if they're wise, should also act on smaller, yet still important, efforts that Copenhagen championed. Hammering out deals to curb deforestation and spread clean-energy technologies would be valuable on their own but would also help reverse the popular sentiment that the Cancun talks are a pointless exercise.

Protecting Copenhagen's achievements will not solve the climate problem—not by any stretch. Nor will it change the fact that climate will increasingly be dealt with outside the U.N. talks. But the U.N. talks can't be avoided—and the Copenhagen Accord, properly reinforced, can provide a core foundation for future efforts to address climate change. Without it, the world is in for an even rockier ride.